The present invention relates to a circuit for controlling the drive of a motor, which is stable with respect to the changes in temperature.
FIG. 1 shows a general speed control system for motors. That is, the speed control system forms a loop in which when a motor 1 is running, a voltage proportional to the running speed is obtained by a speed/voltage converter 2, the obtained signal and a voltage of a reference signal source 3 are compared with each other by a comparator 4, the compared output is amplified by an amplifier 5, and the amplified output is used to drive the motor. In order for the motor 1 to run at a predetermined speed, however, an offset must be given to a differential amplifier constituting the comparator 4. In practice, however, when there was an offset in the input, the differential amplifier could not equalize the temperature characteristics of base-emitter voltage V.sub.BE which was dependent upon a collector current, whereby the temperature drift was developed causing the running speed of the motor to be greatly varied.